Field of the Invention
The invention relates in general to a low-voltage differential signaling (LVDS) driving circuit, and more particularly to a voltage mode LVDS driving circuit.
Description of the Related Art
Low-voltage differential signaling (LVDS), providing good performance as well featuring advantages of low power consumption, low noise, low electromagnetic interference (EMI) and low costs, is extensive applied in high-speed data transmission. FIG. 1 shows a schematic diagram of a conventional LVDS transceiving circuit. A transmitter (or referred to as an LVDS driving circuit) and a receiver of the LVDS transceiving circuit are bordered by the dotted line in the drawing, and the part located on the left side of the dotted line is the LVDS driving circuit. The LVDS driving circuit transmits signals to the receiver at the right side of the dotted line via transmission lines 140 and 145. The LVDS driving circuit includes a current source 110, a switch 122, a switch 124, a switch 126, a switch 128, another current source 115 and a resistor 130. The four switches 122, 124, 126 and 128 may be implemented by p-type metal oxide semiconductor transistors (to be referred to as PMOS) and n-type metal oxide semiconductor transistors (to be referred to as NMOS). In this example, the switches 122 and 124 are implemented by PMOS, and have respective sources coupled to the current sources 110, respective gates as control ends, and respective drains coupled to the switches 126 and 128, respectively. The switches 126 and 128 are implemented by NMOS, and have respective gates as control ends, respective sources coupled to the current source 115, and respective drains coupled to the drains of the switches 122 and 124, respectively. A connecting node of the switches 122 and 126 and a connecting node of the switches 124 and 128 serve as two output ends (respectively coupled to the transmission line 140 and the transmission line 145) of the LVDS driving circuit, and a resistor 130 is coupled between the two output ends. Operations of the LVDS driving circuit are divided into two stages. In the first stage, the switches 122 and 128 are turned on, and the switches 124 and 126 are turned off. At this point, the current lout flows towards the direction indicated by the arrow as shown, undergoes alternating-current coupling at the coupling capacitors 150 and 155 at the receiver, and generates a cross voltage VOD at the load resistor 160. In the second stage, the switches 124 and 126 are turned on, and the switches 122 and 128 are turned off. At this point, the current passing the resistor 130 and the load resistor 160 changes from flowing downwards to flowing upwards, hence generating a different cross voltage VOD at the receiver. The receiver may then learn the information transmitted from the transmitter according to the change in the cross voltage VOD.
The resistor 130 is a matching resistor of the LVDS driving circuit. Further, because the driving circuit is driven by the current source 110 and the current source 115, the resistor 130 and the load resistor 160 of the receiver are in a parallel connection and both having a resistance value R. If the resistance value is 100Ω, the equivalent resistance value is 50Ω when the resistor 130 and the load resistor 160 of the receiver are in a parallel connection. Assuming that the cross voltage VOD of the load resistor 160 needs to be 400 mV, the current lout of the LVDS driving circuit needs to be 400 mV/50Ω=8 mA. That is to say, due impedance matching, the LVDS driving circuit is required to output a large current in order to drive the receiver. Further, as the current source 110 and the current source 115 need to be driven by a larger voltage, the LVDS driving circuit requires a higher voltage VDD, e.g., 2.5V or 3.3V. A drawback of using a high voltage VDD not only increases the overall power consumption (VDD×Iout) of the driving circuit, but also causes the switches 122, 124, 126 and 128 to adopt large-sized components for withstanding a higher operating voltage. For example, I/O devices, whose channel length is usually between 450 nm and 550 nm, need to be used. Such large-sized components indirectly cause a front-end circuit (e.g., an inverter) of the LVDS driving circuit to encounter a larger load, such that the current consumption of the front-end circuit and power noise are both increased.